SoundPool: Avoid busy waiting during stream restart

A stream on the restart queue can cause the StreamManager to
busy-wait.  This was introduced by a fix
commit ba04dbe7.

Test: Verbose log on StreamManager.cpp, run SoundPool CTS tests.
Bug: 182923919
Change-Id: Iae794bc957869426a4e1e27cd3c088aa9dd83208